migrate to python3

[flatmax]

buildroot no longer supports python2 building - this requires the host machine to install python2 to make.sh.

Can you please migrate to python3 support ?

That would simplify issues reported against buildroot.rockchip

[RadxaYuntian]

Please tell us which branches are affected by this. For RK3399 we are now using mainline U-Boot in our systems. Rockchip also provides buildroot support so I can also check if their latest version has migrated to py3.

[flatmax]

I am working with the rock 3a and the cm3 boards

[ericwoud]

I just do:

sed -i '1c#!/usr/bin/env python3' arch/arm/mach-rockchip/decode_bl31.py

It changes the first line to:

#!/usr/bin/env python3

Before building... That's all it needs (for rock5b),